Process of extracting gold



PROCESS OF EXTRACTING GOLD John W. Britton, Lakefield, Ontario, Canada, assignor to Lakefield Research Limited, Lakefield, (Enter-lo, Canada No Drawing. Application April 19, 1957 Serial No. 653,741

Claims priority, application Canada July 3, 1956 Claims. (Cl. 75-113) This invention relates to a process for extracting gold from iron-containing gold-bearing materials including ores, concentrates and residues.

In the treatment of many gold-bearing materials such as ores, concentrates and residues by heretofore known methods, a considerable proportion of the gold is not recovered. This is particularly true of gold associated with such minerals as pyrite and arsenopyrite and other materials especially those containing sulphur, tellurium, arsenic, antimony or lead compounds.

It is common practice to concentrate, if necessary, and roast such materials and then dissolve out the gold, usually by treatment with a cyanide-containing solution. However, it is frequently found that a substantial proportion, for example from 10 to of the gold remains undissolved after such treatment and the residues contain a valuable proportion of gold.

An .object of this invention is to provide a process for recovering a high proportion of the gold from such residues.

A further object of the invention is to provide a method for the treatment of iron-containing gold ores and their concentrates whereby substantially higher recovery of gold is obtained in the subsequent cyanide or other dissolution extraction. The method not only increases the amount of gold capable of being extracted but also increases the rate of dissolution of the gold, decreases the consumption of reagents and reduces the capital and operating costs of the extraction plant.

As applied to the above described residues the invention resides in heating the residue from a roasted and subsequently cyanided iron-containing gold-bearing material to a temperature higher than the maximum roasting temperature to effect a structural change in the particles thereof and thereafter Cooling and treating the cooled residue with a gold-dissolving reagent. It is believed that, prior to this thermal treatment, the gold is held in solid solution in the iron oxide and is incapable of being dissolved by normal methods; as a result of the thermal treatment a phase change takes place in the iron oxide whereby the gold is liberated in a form in which it is capable of being dissolved. If the residue contains appreciable amounts of unoxidized sulphur, arsenic, antimony, lead or tellurium compound it should first be roasted by any suitable method at a temperature from about 500 to 1600 F. before subjecting it to the thermal treatment described herein.

As applied to ores and concentrates the invention resides in roasting the iron-containing gold-bearing mate rial and thereafter raising the temperature of the roasted material to a higher temperature than the roasting temperature to efiect a structural change in the particles of the material. In treating such processed material to dissolve gold as by cyanidation it has been found that a subsfaptially higher percentage of the total gold is recovered atent ice in a shorter period of time and with the consumption of less cyanide than has heretofore been possible by other methods.

The effective heating temperature for the treatment of said residues will vary according to the composition of the material being treated and will exceed 1000 F. It will be observedfrom the examples to follow that from 1100 F. to about 1750 F. the percentage yield of gold from the residue varies almost directly with the temperature. The yield begins to rise rapidly as the temperature is increased from 1750" F. to about 2000 F. Above this temperature the yield begins to fall but is still appreciable even at a temperature of 2400 F. The reduction in yield obtained at temperatures about 2000 F. is believed to be due to partial fusion of the material, which fusion tends to counteract the liberating effect of the higher temperature. Excessively high temperatures also cause wastage of fuel, increased wear on the furnace materials and loss of gold due to volatilization. The preferred temperature range therefore will usually be from 1650 F. to 2200 F. and more particularly within the range from 1800 F. to 2150 F.

The following examples illustrate typical results obtained by the method of the invention as applied to said residues.

Example 1 A sample of residue resulting from the roasting and cyanidation of a gold-bearing ore was placed in a refractory crucible which Was covered with a l0oseiy-fitting fireclay lid. The residue before treatment contained 0.75% arsenic, 0.13% antimony, 0.02% lead, 0.02% sulphur present as sulphide, and 0.126 ounces of gold per ton. The crucible was heated in a mutlle furnace to a temperature of 1967 F. and was then removed and allowed to cool. The product was agitated for 24 hours with a solution of sodium cyanide and lime. The pulp was then filtered and the residue was washed. T he solution was then assayed and the results showed that 41.6% of the gold in the original residue, equivalent to 0.052 ounces per ton had been extracted. At a mine treating 1000 tons of similar residue per day, the increased gold extraction resulting from the use of the new process would amount to 52 ounces per day. With gold at 35 dollars per ton, the value of this additional gold would exceed 1800 dollars per day.

Example 2 A sample of calcine residue containing 0.99% arsenic, 0.13% antimony, 0.52% lead, 0.71% sulphur present as sulphide, and0.70 ounces of gold per ton, and resulting from the cyanidation of a roasted gold-bearing concentrate, was heated for 30 minutes at a temperature of 2012 F. in a refractory crucible which was covered by means of a loosely-fitting lid. At theend of the heating period the crucible Was removed from the furnace and allowed to cool. The product had calced slightly during the treatment and was therefore crushed to pass a 65 mesh screen. It was then agitated for a period of 24 hours with a solution of sodium cyanide and lime. At the end of the cyanidation period the mixture was filtered and the residue was washed with water. An assay of the filtrate and washes showed that 67.6% of the gold, equivalent to 0.473 ounces per ton, had dissolved. At a mine treating say tons of calcine per day, and with gold at 35 dollars per ounce, the value of the gold recovered by means of the present process would exceed 1600 dollars per day.

The following table shows the effect of the heat treatment of the same calcine residue at the temperatures indicated:

Gold extracted by cyanidation Temperature during thermal treatment 1 Percent Ounces *per ton The following are experimental results obtained during a study of the effect of .certain variables on the gold extraction obtained from the same calcine residue. Although these results may be considered as typical they will not necessarily apply in the case of materials of different compositions.

EFFECT OF VARYING HEATING PERIODS Test No N M 7 Temperature l. Time charge held at temperatur Gold extraction EFFECT OF STIRRING DURING HEATING career or osrNo A rnnnnrlznn cannon" Test No; or M Temperature 2,012 F"-.- 2,012 F Type of charge Powder Pellets. Time held at temperatur minutes 30 minutes Gold extraction r *This test was carried out on the residue from test ,N

EFFECT OF GRINDING The residue from test M was ground to 95% minus 325 mesh and was recyanided. An additional recovery of 4.2%'was obtained. a a

EFFECT OF OOOLI N G AT DIFFERENT RATES Test P-Fairly rapid cooling: crucible containing charge was removed from furnace and allowed to air-cool, Test Q'Rapid cooling: the charge was quenched by pouring into Water. Test R 'Slow cooling: the charge was left in the furnace and allowed to' cool slowly.

Test No P Q R Type of cooling-.. Fairly rapid..- Rapid"..- Slow. 1 Temperature--. 1,967 F l,967 F, 1,967 F. Gold extr on 7 V 60.9% 7 D 55.9%".-. ..57.6%.'

' lead and 5.13'ounces of gold per ton was divided into rendered capable of being dissolved.

From these results it may noted that optimum results 7 were achieved when the residue was pelletizedgheldwitb out agitation at the heating temperature for about 30 minutes, cooled fairly rapidly and finely divided before cyanidation.

In the treatment of ore, concentrate and other not previously roasted gold bearing material it is first roasted by any suitable method such as in a fluidized bed roaster or in an Edwards-type roaster. The roasting temperature may vary from about 500 to 1600 F. .Itis desirable that the material be roasted at a-relatively low tempera ture initially and raised relatively slowly to a higher roasting temperature as is wellestablis'hed inth'e 'art. The roasting time will vary with "the-composition of material being roasted and with the -temperature used, and. will range from about 1 to 10 hours. It is preferable that the roast be conducted within from 1 ='to 4 hours and with a maximum roasting temperature between 900 and that, prior to the high-temperaturetreatment, part Of'i h:

gold is held in solid solution in iron oxide and is incapable of being dissolved by normal methods. As a result of .the high temperature treatment a phase change takes place in the iron oxide whereby gold .is liberated and It h b en :f un that the gold extraction obtained in the subsequent dissolving step increases with increasing temperature 11p zto a certain maximum extraction or optimum temperature beyond which decreased gold extractions are obtained. This optimum temperature varies with the composition of material being treated but is usually between '1650 and 2200 F. Above the optimum temperature, although the extraction is inferior to that obtained by heating at the optimum temperature, there is still a temperature range in which better extractions are obtained thanvcan be obtained from calcine which has not been'subjected to this process. Although the preferred temperature is with in the range from 1650 to 2200 F., andamore particularly within the range from 1800 to 2150" R, increased yields can be obtained by heating at any temperaturebet-ween the'maximum reached in the previous roasting process 2200 F., and in the case of certain materials a the scope of this invention temperatures as high as 240i) F. will give increased yields as compared with those obtained Without the use of this process. V 1

The heated material is cooled and then treated to solve the gold. Currently cyanide in alkaline solution (usually lime) is normally used by chlorination or any other suitable extractive process may be used.

It has been found that grinding the heated and .srmc.

turally changed material before cyanidation, as is shown in the examples below has a beneficial efiect on the yield.

The material is preferably finely divided so that at least passes through a 200 mesh screen. Also pelletizing the charge of material before subjecting it .to this'heat treatment has a beneficial effect on the recovery. Holding the material at the heating temperature for a shortjperiod gives a somewhat increased yield. A second limit-treatment and cyanidation gives a small increase in yield butwould not be economical in all'cases.

The following examples demonstrate the improved results obtained by using the new centrates:

Example 3 .A sample of gold-bearing. concentrate containing 20.78% sulphur, 10.72% arsenic, 0.85% antimony, 0.22%

equal parts, two of which were placed in fireclay which were then transferred to a muflle 'furnace'which process on ores and-con- V had been preheated to 400 F. The temperature of the under conditions similar-to those used for treating the furnace was then raised slowly to 900 F. and maintained first sample. Results of the tests were as follows: for 15 minutes, after which it was increased to 1300* F. The charges were rabbled at frequent intervals in order D to ensure eficient oxidation. As soon as the temperature g g g a1 gigg y of 1300 F. had been reached, both dishes were removed. roast) method of One was allowed to cool and the second dish was covered treatment) with a loosely-fitting fireclay lid and transferred to a seceee e ee e eemeeeeteee ef 1300 The 22:3333e33333333333a3 33333elie; 3333 3: 333 perature was raised to 1967" F. and the (11511 was then Gold extraction, percent of total gold 92.2 96.3 removed and allowed to cool. After taking a sample of 30mm cyanide consumed 34cm 14 each for analysis, the calcines were ground to 95% passing 325 mesh in the presence of a solution of sodium cyanide These figures 5h0W that, for}! mine @eatlng 1000 T0115 and lime and the pulps then agitated in open bottles on P y 0f Simllar 0T6, the Increase In gold recovery rollers f r a i d f 43 hours, A the d f i 15 resulting from the use of the new process would amount period the pulps were filtered and the washed residues to abmlt 30 ounces p y- Wlth gold at 35 dollars were recyanided for two successive periods, each of 16 P I011, the Value of the extra gold recovered would hours. The various cyanide solutions and final residues ceed 1000 dollars P ywere then assayed for gold. Results of the tests were Example 5 as follows: 20

A test was carried out as in Example 4, but the thermal treatment was done at 1800 F. instead of 1975 F. The A B comparative results were as follows: (Product (Product from normal from new roast) method of 2.5 Heating Temperature treatment) 1,s00 F. 1,975 F. Assay of calcine before cyanidatlon, oz.

gold/ton 6.23 6.52 Assay of final residue, oz. gold/ton 0.652 0.345 Assay of calclne before cyanrdation, oz./ton 0. 788 0.800 Assay of cyanidation residue, ozJton 0.040 0.030 Gold extraction (percent of total gold in cal Percent of total gold in cine) 94. 9 96. 3

the calcine Sodium cyanide consumed (per ton of ore),

pounds 0. 14 0. 14 Gold extraction:

1st period (48 hours) 85. 1 93.4 period (lfihours Similar comparative results of concentrate treatment 3rd period (16 hours) are as follows:

Total (80 hours) 89. 6 94. 8

Sodium cyanide consumed (per ton of Heating Temperature original concentrate), pounds 2. 30 1. 52

d l d 1,800 F. 1,950 F. Almost all the cyanide-soluble gold was isso ve in the first 48 hours treatment of test B calcine, whereas 1an 362E135 233%,;liffieilfif fiifihn'itf'i iII: 833g 8: additional 16 hours was required to achieve a simiar t on, p rcent 90.7 result in test A, thus demonstrating the higher rate of ifi gfi ilii lf fiiiijgfffifi extraction obtained as a result of the new process.

Calculations show that, at a mine treating 100 tons of The examples illustrate the increase in yield which Similar Concentrate P5 l the Increase gold extrac' is obtained by this heat treatment of the roasted material. lion resulting from this Improved method of trfn'atment The increase-is particularly striking when translated into would amount to about 26 ounces P P Wlth gold dollars per day. Further the high rate of gold extraction Valllfid at 35 donafs P Ounce, that Increased recovery 50 enables smaller cyanidation plants to be used, thus effect- WOllld be Worth more than 900 dollars P ying considerable savings in the capital and operating costs of the extraction plant. Additional savings also result Example 4 from the reduction in cyanide consumption.

This application contains subject matter in common A sample of ore containing 3.25% sulphur, 1.73% with my 653742 filed April 1957' arsenic, 0.13% antimony, 0.04% lead and 0.745 ounce What 15 clamwd 1s: of gold per ton was roasted in a mufile furnace, the temi a.process ramming gold upn'comalmng perature being raised gradually to 1000 F. The furnace go earlng material p the mat enal ls roasted a was held at this temperature for 30 minutes and the t,reated.WIth gold'dlssolvmgfeagept tne Steps of heating temperature was then raised to 1300" F. After maintainme resldue from the gol'dlssolvmg.treatment to a ten?- ing this temperature for 60 minutes the charge was allowed perature at least 1650 b hlgher than h maxito cool slowly. The charge was stirred at frequent inmum Feasting temperalure cpohng the heated resldue and tervals during the roasting period. A sample of the retreangg Wlth gold dlsso.lvmg.reagent calcine was analysed and was found to contain only 0.02% h g 6 process defined m claim 1 Wherem i remdue sulphide sulphur, showing that oxidation of the sulphides g g temperdatllredn9t q than 240.0 was virtually complete. A portion of the calcine was is g g g iz ggf 2 1 i i gs is g ground to approximately 95% minus 325 mesh and was not more than 22600 F e no 658 an an g d fop a pe1l1od of hours; tne pulp W l' 4. The process defined in claim 1 wherein the residue and toe rcsldll wa Washed Water bemg 70 is heated at a temperature not less than 1800 F. and dried and assayed. A further portion of the calcine was not more than F placed in a refractory crucible Wh1ch was then covered 5, In the removal of gold from iron-containing goldwith a loosely-fitting lid and heated in a furnace at a bearing material wherein the material is roasted and temperature of 1975" F. The crucible was then allowed treated with gold-dissolving reagent, the method which to cool, after which the calcine was ground and cyanided comprises pelletizing the residue from the gold-dissolving Eastman; heating 'the pe'llete'd residae a a temperatute'ef at least 1650" F1 hut higher than the maximum roasting temperature, cooling the heated residue and treating it treatment to atemperature' of at least 1650 F. but higher" than the maximum roasting temperature, cooling and finely dividingthe so heated residue to'cause at least 90% of it to pass through a 200 mesh screen and treating it witli' a gold dissolving reagent;

' -7, process for treating unroasted iron-containing" gold' bearing material which comprises roasting, the material and immediately. thereafter heatingthe roasted ma tetial" at a temperature higher than that atiwhich it was roasted and not less than l650 'n'or more than 2400" F. 8. The process defined in: claim 7 wherein the heating temperature is not more than 2200 F. 9; The'proc'e'ss defined-incIaimT wherein the heating temperature is not less than 1800 F1 not more than 2150F. T U

' 101 A process for treating unroasted iron-containing gold hea'ring ore's'which comprises roasting the ore at a maximum temperature between 900 and 1600" F., immediatel'ythereafter'heating the roasted material to a temperature between 1650' and 2'200 1f F. and treating the heated material to dissolve the gold therein.

11. A process as defined in cla'im 10:wherein' the heat ed material is finely dividedbefore" dissolution treatmefit to such fineness that at-least-90-% will pass through a200- mesh screen.

1 21 Aproeess'a's defifned in claim 1:1 wherein the roasted material} is pelletized before heating.

1 3 A process for treating unroasted iron-containing gold-hearing materials" which comprises roasting the material; pellejtizin'g the' roasted material, heating the pelletized material at atemperature between 1800 and 2150 F.,' finely dividing the cooled material to at least 200 mesh and tr'e'atingthe finely-divided material in cyanide solution.

141 A processes defined in claim 13 wherein the resi due after cyanidetreattnent is heat-treated at a temperature higher than that used in the previous heat-treatment, but not greater than 2400 E, and treating the heat v treated material Witha cyanide-containing solution.

'15. In the process of recovering gold from an ironcontaining v gold bearing materiali involving roasting the material and thereafter leaching with' a cyanide-containing'solution, the improvement:which'consists in subjectingthe'materiaLsubsequent.to a. roasting treatment, to heat treatment at a temperature within the range 1650 References Cited in the file of this patent UNITED STATES PATENTS Great Britain Apr. 19, 1950 

6. IN THE REMOVAL OF GOLD FROM IRON-CONTAING GOLD BEARING MATERIAL WHEREIN THE MATERIAL IS ROASTED AND TREATED WITH GOLD-DISSOLVING REAGENT, THE METHOD WHICH COMPRISES HEATING THE RESIDUE FROM THE GOLD-DISSOLVING TREATMENT TO A TEMPERATURE OF AT LEAST 1650* F. BUT HIGHER THAN THE MAXIMUM ROASTING TEMPERATURE, COOLING AND FINELY DIVIDING THE SO HEATED RESIDUA TO CAUSE AT LEAST 90% OF IT TO PASS THROUGH A 200 MESH SCREEN AND TREATING WITH A GOLD-DISSOLVING REAGENT 